Releasable mechanical fasteners are widely used for a vast array of products and applications. One class of such fasteners is the hook-and-loop variety, which include one part having multiple hook shaped projections, and another part presenting a large number of loose loops of fabric or fiber intended to snag on the hooks. Such fasteners have found particular success in e.g. garment manufacturing, and are commercially available under e.g. the Velcro™ brand-name from the Velcro USA Inc and under the SCOTCHMATE brand-name from Minnesota Mining & Manufacturing Co. of St. Paul. More recently releasable fasteners that are self-mating, e.g. where both parts of the fastener are identically constructed and mutually interengage, have become available. One such example is the Dual Lock™ reclosable fasteners commercially available from Minnesota Mining & Manufacturing Co. of St. Paul, Minn. Both types of reclosable fasteners can be fabricated by preparing a web having projecting stems thereon (herein referred to as “stem web”) and then capping those stems to form an array of shaped projections. The stem webs typically have an indefinite length and given width. The capping of the stems is done against a heated roll as described in U.S. Pat. No. 5,679,302, the entire contents of which are hereby incorporated by reference. A limitation with this process is that the gap between the rolls must be precisely controlled in order to maintain the final thickness of the product and the size of the caps. Another limitation is that the process is only suitable for relatively lower line speeds.
A more recent development in capping processes for stem webs was the invention of a continuously tapered nip or capping shoe. Such an apparatus is described in U.S. Pat. No. 6,039,911, the entire disclosure of which is hereby incorporated by reference. The main advantage of this design is that more contact with the heated roll allows the process to be run at higher line speeds. However, it was found that difficulties were encountered when extrapolating the method to wider web widths. The method involves a variable nip, and the exact dimension of the gap at that nip is very important. At wider web widths, roll deflection due to mechanical bending or non-uniform temperatures makes it difficult to achieve a gap that has a uniform dimension across the entire roll. This can be partially balanced with larger roll diameters, but that leads to other problems with weight, cost and thermal control.
More recently then, expedients adapted from roll processes used for the calendering of paper, plastic and magnetic media have been used for the making of self mating fasteners. These processes stack several rolls together so that there are loads on both sides of most of the rolls. In this process the capping is accomplished in a series of incremental steps, which lowers the peak force required in each step and lowers the amount of roll deflection.
FIG. 1a shows a conventional calendering process for capping stem webs. In FIG. 1a, the stem web 10, having a backing 12 and stems 14 thereon, is maneuvered over idler 16 and directed into a nip 26 between heated roll 20 and cooled roll 24. The stem web 10 is then directed through a second nip 28 between cooled roll 24 and heated roll 30. The stem web 10 then passes through a third nip 32 between heated roll 30 and cooled roll 34, and then a fourth nip 36 between cooled roll 34 and heated roll 38. The stem web 10 is then directed to a fifth nip 40 between heated roll 38 and cooled roll 42. The stem web 10 is then directed to a sixth nip 44 between cooled roll 42 and heated roll 46. Finally, the fully capped web 48 is drawn off. It should be noted that because heated roll 20 at the top and heated roll 46 on the bottom are loaded on only one side, they have a larger diameter to minimize deflection. This process has been successfully used to fabricate HookIt II brand abrasive sanding disks commercially available from Minnesota Mining & Manufacturing Co. of St. Paul, Minn.
Referring now to FIG. 1b, a detail side view of a portion of fully capped stem web 48 according to conventional processes requiring three or more nips to finally cap the stem web. The figure is used to define reference dimensions for capped stem webs. In this view, one stem 14 is seen in isolation, extending from backing 12 and having a diameter “d”. The use of conventional processes, such as that disclosed in FIG. 1a, has stem 14, forming cap 50, resulting in capped stem 52. The cap 50 has a diameter “D”. A convenient way of expressing the degree of capping achieved is the ratio of D:d. For the Hook-it™ II product, a D:d ratio of approximately 1.66 is achieved at the end of the process, and this ratio is found to give the desired strength of the bond with loop material. The method results in capped stems 52 having a good symmetrical shape without buckling, and a consistent result across the width of the capped web 48 if the web width is less than 1 meter and the line speed is less than about 30 m/minute. However, the complexity of this mechanism, and the difficulty of properly gauging six distinct but interacting nips have proven to have disadvantages. Also, the process is proved to be unwarrantedly difficult when attempting to extrapolate to larger diameter stems and faster line speeds. But simpler methods have proved elusive; for example, with the process according to FIG. 1a, D:d ratios of only about 1.44 being achievable at second nip 28. The art still requires a method for capping stem webs that is simple, usable with wider web widths, and capable of running at higher web speeds even when processing stem webs with stems of greater diameter.